Over the years, since the invention of integrated circuits, an increasing number of high voltage circuit functions have been integrated into the silicon integrated circuit. Prior to this the high voltage circuit functions were implemented with discrete components or designed into hybrid modules. These two technologies are expensive for a given circuit compared to an integrated circuit.
The present invention provides an alternative to existing arrangements capable of isolating delicate components in the circuit from static discharge damage which may be of the order of kilovolts.
An important feature for implementing high voltage functionality on a semiconductor integrated circuit is to isolate the core circuitry behind high value resistors, usually of poly silicon resistors. Unfortunately, a problem arises when the chip is subjected to electrostatic discharges, ESD, because the resistance offered by the resistors is much higher than the output resistance of the ESD discharge. This causes a significant voltage to appear on the integrated circuit. Since ESD voltages are typically a few kilovolts, damage to the field oxide of the circuit may result. A particularly difficult problem arises when an input pad has to support both positive and negative high voltages in normal operation. Under these conditions, it is unlikely that a suitable on-chip diode pair can hold off the operating voltages and protect the semiconductor chip from ESD damage.
In principle, a simple spark gap may be used to provide protection for either polarity pulse and also to hold off the circuit voltages. A spark gap can be made to operate at less than 1000V on an integrated circuit, which may be adequate to protect the field oxide.
However, a further complication arises from the commercial need to use inexpensive plastic encapsulation for the silicon chip.
The present invention thus provides a spark gap operable in a plastic package, and a protection device for operation at about 2 kV ESD voltage (human body model, HBM).